Frequently, more than one fan is installed for cooling internal combustion engines, the fans being controlled via a common control unit. The fans may be controlled in almost any manner with the aid of a pulse width modulation system for the purpose of setting the extent of the power consumption of the blower. Each of the fan motors is connected to a common supply line via a switching device, preferably a field effect power transistor. The field effect power transistors are activated periodically with the aid of the pulse width modulation signal, a pulse control factor of the pulse width modulation signal specifying for which component in time of a signal period the field effect power transistor is switched through or blocked.
The periodic switching of the field effect power transistors causes voltage fluctuations on the supply line, which may have an interfering effect on the entire supply network. In order to reduce these line-connected interferences, usually a capacitor in the form of an electrolytic capacitor, is provided, which forms a low pass filter on the supply line, and which thus smoothes out the voltage fluctuations on the supply line.
Because of the switching of the field effect power transistors, the capacitor is loaded by an alternating current. The loading is essentially a function of the motor currents and the pulse control factor. Each of the fan motors causes an alternating current flow in the capacitor, as a function of the respective pulse control factor of the pulse width modulation signal by which the respective fan motor is controlled.
The alternating current flow through the capacitor loads it, the capacitor generally being heated by the current. Especially in the case of the usual application of an electrolytic capacitor, the service life of the component is considerably reduced, since, for example, the electrolyte of the electrolytic capacitor may dry up.